I Worked on one with the exact same issue, check the regulator that's mounted on the side of the chassis. Also check the bridge rectifier that's mounted off the power supply board circuit card with the heat sink. (I think it's part of the switching and acc power supply setup but can't remember off the top of my head.) Also check the filter caps for that specific section as well. I chased this issue down and in the end after chasing all said above it was a shorted power tranny, was not fun tracking down and taking apart several times.